The present invention relates to vibration systems such as repetitive shock (RS) vibration systems. An RS vibration system may be a six degree-of-freedom table movably mounted on a base and provided with a series of impactors or vibrators configured to impose a desired vibration on the table (e.g., for stressing one or more products mounted on the table during a product test cycle).
In some known vibrator systems, one or more accelerometers are provided on the table and used to influence a control signal for the vibrators in the system. As described in U.S. Pat. No. 6,220,100, the outputs of multiple accelerometers mounted across the table are averaged and the average output is used as a control parameter for setting a single control signal sent to all of the vibrators. However, this concedes that the vibration response at various points on the table is uneven, necessitating the averaging of multiple accelerometer outputs to produce a useful control signal. While this approach can have the effect of centering the vibration response of multiple vibrators about a target, it will not tighten the overall dispersion since it is limited to providing one all-encompassing control signal to all the system vibrators, which may not have identical output when stimulated with identical control signals.
In another known vibration system, disclosed in U.S. Pat. No. 7,752,914, individual vibrator performance is accounted for by monitoring air pressure values, for example, between each vibrator and a corresponding valve upstream of the vibrator. With this information, the control system can account for variance in vibrator efficiencies so that all the vibrators in the system operate with substantial uniformity.
In another known vibration system, vibrator control valves are provided for the vibrators provided on a vibration table. During an initial test setup procedure, a dummy load can be placed on the table, along with several accelerometers. The vibrators are operated and vibration strength data is collected from the accelerometers. Fixturing, load placement, and accelerometer position are adjusted iteratively by a highly skilled operator to get a coarse adjustment for uniform vibration response. When reasonable results are achieved, adjustment factors are determined for each of the vibrator control valves in an attempt to fine-tune the system for its best possible uniformity or vibration response for the particular load scenario. Then the test is run, using a single control signal averaged from multiple accelerometers, with the corresponding preset adjustment factors being applied to the vibrator control valves throughout. The test must be stopped for re-calibration of the system to correct any changes or degradation in performance.
Over the years, considerable resources have also been spent by those of skill in the art to develop tables with increased vibration uniformity through complex structures. Despite advances through mechanical development, one or more of uneven table loading (by the product or products being tested), variance in individual vibrator efficiencies, and variance in transfer efficiencies between individual vibrators and the table can cause a significant disparity between the desired vibration strength across the table and the actual vibration strength across the table when utilizing a control system with the limitations of those currently known in the art.